Membranes containing physical pores have been used in a variety of separations such as filtration, microfiltration, ultrafiltration, and reverse osmosis. The size of the pore structure in the membrane is an important factor in determining the type of separation in which the membrane can be used. Membranes with pore sizes greater than about 10 .mu.m are generally used for filtration, while microfiltration utilizes membranes with pore sizes in the range of 10 .mu.m to 0.1 .mu.m and ultrafiltration utilizes membranes with pore sizes less than about 0.1 .mu.m.
To maximize flux through microfiltration and ultrafiltration membranes, it is advantageous to make the layer containing the small (less than about 0.25 .mu.m) pores as thin as possible. One way of achieving this is to construct an asymmetric membrane structure in which a small pore layer is composited with layers having larger pores. The small pore layer is the active micro or ultrafiltration membrane, while the larger pore layer or layers provide mechanical strength without significantly reducing the flux through the membrane.
Asymmetric membrane structures are routinely made from polymeric materials. By controlling the casting and drying conditions, it is possible to obtain a thin, small pore layer on one side of a sheet which has large pores. Polymeric asymmetric membranes have been extensively used in ultrafiltration applications; however, polymeric membranes can only be used in low temperature (&lt;150.degree. C.) applications because of their thermal unstability and they cannot be cleaned with strong acids, bases and oxidizing agents once they have been fouled because of their chemical reactivity.
To overcome these limitations, some researchers have been experimenting with asymmetric membrane structures made entirely from ceramic materials. These asymmetric inorganic membrane structures have several attractive features including: (1) high temperature stability, (2) an ultrafine pore structure in the asymmetric layer which can be fabricated in sizes ranging from about 5 .ANG. to 2500 .ANG., and (3) relatively high permeabilities due to the thinness of the asymmetric layer. Currently, modules made using these membranes have not been extensively utilized in industrial processes because of the relatively high cost per unit active area and because of issues associated with the possibility of catastrophic failure of the ceramic elements.
The present invention seeks to provide structurally sound micro and ultrafiltration membranes. Another object of the present invention is to provide a new thermally and chemically stable membrane structure.